Project Summary The dimorphic fungus, Histoplasma capsulatum (Hc) is found world-wide, and endemic to the Midwestern and southeastern United States. The organism can cause a life- threatening infection in immunocompetent or immunosuppressed individuals. We have shown that the absence of chemokine receptor, CCR5, enhances the ability of mice to clear infection with Hc. Accelerated resolution is associated with a perturbation in the balance between regulatory T cells and interleukin (IL)-17+ cells. Mice lacking the CCR5 receptor manifest a decrement in the number of regulatory T cells in lungs and higher levels of IL-17. The paucity of regulatory T cells is multi-factorial. These cells do not emigrate from the thymus, and they proliferate poorly in the lungs. Moreover, we have demonstrated that the absence of CCR5 attenuates the deleterious effects of tumor necrosis factor (TNF)-a antagonism. Herein, we will explore the mechanisms by which the lack of CCR5 signaling is important in disturbing the balance between regulatory T cells and IL-17. Specific aim 1 will investigate the specific mechanisms that lead to the imbalance between regulatory T cells and IL-17. We will produce CCR5-/- mice that express Foxp3-green fluorescent protein (GFP) and IL-17-GFP to track the migration of specific cells populations. We will investigate why regulatory T cells do not proliferate and why they fail to migrate from the thymus. We also will analyze the functional attributes of these cell populations in vivo. In specific aim 2, we will analyze the cellular and molecular effectors that enhance fungal clearance. We will investigate the role of CCR5 ligands in dictating clearance, the direct and indirect influence of IL-17 and the roles of transforming growth factor-b and IL-23. In specific aim 3, we will define the mechanisms by which the absence of CCR5 is salutary to mice lacking TNF-a. We will 1) determine the trafficking of regulatory T cells and IL-17+ cells in mice lacking TNF-a, 2) elucidate the role of specific molecular effectors, and 3) test if CCR5 inhibitors mimic the effect of the absence of CCR5. The goal of these studies is to better understand the host defense mechanisms that are activated to combat Hc. The studies investigating the salutary effect of the absence of CCR5 in mice lacking TNF-a are particularly germane to reports of progressive histoplasmosis in patients receiving TNF antagonists. Our findings will contribute to a greater understanding of how the host can successfully defend against histoplasmosis.